1. Field of the Invention
The present invention relates to a conductive structure for a semiconductor integrated circuit (IC). More particularly, the present invention relates to a conductive structure being capable of preventing oxidation of the surface.
2. Descriptions of the Related Art
Bump technology has been widely adopted in some fields, such as microelectronics and micro systems, to provide an electrical connection interface between a semiconductor IC and a circuit board. For example, in the connection between a circuit board and an IC chip, the IC chip may be connected with the circuit board in various manners. The bumps are formed within openings defined by a passivation layer on pads of the IC chip, so that the pads are electrically connected to pins of the circuit board.
Among various metal materials, gold has a superior electrical conductivity. As a result, gold is commonly used in the packaging processes as the bump material and is referred to as a gold bump. As shown in FIG. 1, a bump is formed in an opening defined by a pad 101 and a passaviation layer 102. The bump is defined jointly by a first conductive layer 104, a second conductive layer 105 and an under bump metal (UBM) 103. For this example, the first and the second conductor layers 104, 105 can be both made of gold and are formed as one piece. An UBM 103 on the pad 101 will first be formed in a typical gold bump process. The UBM 103 typically is also electrically connected to a conductive layer (for example, the first conductive layer 104 in FIG. 1) to form the bump through an electroplating process, besides being used as an adhesive layer for connecting the bump and the pad 101. The first conductor layer 104 may be substantially formed at a same or a different time with the UBM 103. The first conductor layer 104 and the UBM 103 may also be formed through the same manufacturing process and with the same material, so that the first conductor layer 104 and the UBM 103 can be used as an electrically conductive medium after the bump is formed. In this way, the bump can be formed above the UBM 103 and electrically connected to the pad 101 via the UBM 103. For example, the UBM 103 may be made of a material selected from a group consisting of titanium (Ti), wolfram (W) and alloys thereof.
Since gold is very expensive, composite bumps made of gold and other metals in combination have been developed in the prior art to reduce the cost. For example, in the bump structure depicted in FIG. 1, the first conductor layer 104 and the second conductor layer 105 are made of materials different from each other. According to such an improved design, in this embodiment where the first conductor layer 104 accounts for a higher percentage in the whole bump than the second conductor layer 105, and the first conductor layer 104 is made of metal material having an electrical conductivity slightly poorer than gold.
In some cases, an intermetallic reaction tends to occur between the first conductor layer 104 and the second conductor layer 105 in a solid phase to cause poor electrical conductivity therebetween, thus adversely affecting the electrical conductivity of the bump as a whole. To improve this problem, a buffer conductor layer 106 is further disposed between the first conductor layer 104 and the second conductor layer 105 in some solutions of the prior art, as shown in FIG. 2.
However, both the aforesaid composite bump and the bump with an additional buffer conductor layer have a prominent shortcoming. That is, when the conductor layers are made of a material other than gold, the surface thereof is liable to oxidation, causing degradation in the electrical conductivity of the whole bump and even damaging the whole bump structure.
In view of this, it is highly desirable in the art to provide a solution that can reduce the cost of the bump while still avoiding oxidation of a surface thereof.